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Aalborg UniversityCommunication system for the AAUSAT-II

Communication Systemfor the AAUSAT-II

Kresten K. Sø[email protected]

Department of Control Engineering Aalborg University

mailto:[email protected]

mailto:[email protected]


AAUSAT-II Mission Objectives•Education

•Communication(One-way and two-way)

•Attitude Determination & Control System(Detumbling and pointing)

•Payload (Gamma Ray Detector)•Deploy extra solar panels•Upload new software for radio amateur use


MECH - Mechanical structure

OBC – Onboard Computer

CDH – Command Data Handler

COM – Communication

EPS – Electrical Power Supply

P/L – Payload

ADCS – Attitude Determination and Control System

GND – Ground station

MCC – Mission Control Center

AAUSAT-II Overview


COM SystemPurpose

• To create a communication path between the satellite and Earth.

Demands• Simple, robust design• Power saving• Small • Interface to: CAN, RS232, and Radio


Block Diagram

• Components – MCU– Radio– CAN – UART– Modem


Specifications for COM 1/2Radio Frequency: 437.425 MHz (Half

duplex)6 kHz

bandwidth

Data rates: 1200/2400/4800 bit/sec

FFSK/MSK-modulation

Transmitter Power: 0.5 W

Link budget: (Worst case)Downlink

margin6,2 dBUplink

margin 20,5 dB CAN bus: 125 kbps


Specifications for COM 2/2Mass incl. Antenna: 110gSupply voltage: 3.3 V

Power Consumption Listening: 184.8 mW Receiving:

283.8 mW Transmitting:

2085.6 mW


Modulation options • FSK -

Frequency Shift Keying

• MSK - Minimum Shift Keying

• GMSK - Gaussian Minimum Shift Keying


MSK Modulation• Advantages of MSK modem– Good noise performance compared to FSK– Easy to use

• Disadvantages for MSK modem– Synchronous interface

• Synchronization– Data must be synchronized to clock– The modem synchronize automatically on bit level – Synchronization on byte level must be done in

software on the MCU


Clock Divider• Adds the possibility of bit rates down to 150

bps by dividing the data clock with 1, 2, 4 or 8• Gives more than one symbol per bit and makes

the signal easier to demodulate.


Basic BeaconBasic Beacon format: AAUSATII <data>

• AAUSATII sent as Morse code on FM• An 800 Hz Morse tone is sent to the radio• Makes it easier to find the satellite • Morse tone created with the MCU PWM

generator and a low-pass filter

• Data is modulated by the modem• 6 bit battery voltage + 2 bit counter

After low-pass filter


ProtocolConnectionless AX.25 using UI frames

• Flags used for synchronization • Header for identification of packets• Checksum for verification of packet

contents• Connection orientated AX.25 is to difficult

to implement considering our demands• The connection orientation is placed on a

higher level on the OBC and GND server.


SoftwareSoftware tasks

• Relay data between GND and CDH• Send basic beacon • Collect housekeeping data

Interfaces• CAN bus• USART for modem and radio• UART for GND segment and debugging


Software Structure

Interrupt module• External

communication functions

• Time critical functions

Main module• Internal

functions• Non time

critical functions


SynchronizationSynchronous bit stream without start and stop bits

• Hardware synchronization difficult• The AX.25 flag is used for

synchronization• Synchronization is done by the ISR

101111001001111110011111100111111001

ISR sync.

CD


Pictures – Radio prototype


Pictures – Modem prototype

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